MRI Heart (Cardiac MRI)

Authors: Dr Brett Lorraine*
                            Dr Charles Lott *

What is cardiac MRI?

Cardiovascular magnetic resonance (CMR) is an advanced form of magnetic resonance imaging utilising electrocardiogram gating to avoid cardiac motion blurring.

Its main use is in assessment of anatomy, function and viability of the heart, but it is also useful in detecting ischaemia and infarction, and in assessing congenital heart disease, the aetiology of heart failure, heart valve dysfunction, masses and the presence of inherited diseases.

Advantages

Cardiac MRI offers greater soft tissue detail than does echocardiography, and can provide unique information with regard to scarring, viability and masses. Soft tissue detail is also superior to computed tomography (CT) scanning. It might be used to further evaluate pathology identified by each of these imaging modalities. The absence of ionising radiation is also an advantage, particularly in young patients and in those requiring multiple scans to follow their condition.

Main concerns/limitations

Access to cardiac MRI can vary depending on geographical health service and patient referral policies. For instance, in Australia, cardiac MRI is currently restricted to specialist referral. This and the limited number of specialised cardiac MRI imaging facilities, especially in rural regions, make MRI more difficult to access than more traditional modalities, such as echocardiography and exercise stress tests.

Cardiac MRI is not currently capable of high-resolution imaging of the coronary arteries and quantification of arterial stenoses, for which catheter angiography and CT angiography are superior. It does have an accepted role in assessment of anomalous coronary arteries.

What are the generally accepted indications for a cardiac MRI?

The following are examples of common indications for cardiac MRI studies:

• assessment of congenital disease of the heart or a great vessel;
• assessment of tumour of the heart or great vessel;
• abnormality of the thoracic aorta;
• assessment of myocardial perfusion and viability (including ‘stress’ imaging);
• evaluation of infiltrative diseases, such as sarcoidosis and amyloidosis;
• assessment of diseases of the pericardium;
• exclusion of anomalous coronary origins;
• quantification of cardiovascular shunts;
• quantification of ventricular function.

What are the prerequisites for having a cardiac MRI done?

Recent renal function tests, particularly estimated glomerular filtration rate (eGFR), is of assistance in confirming adequate renal function pre-test, to lessen the chance of NSF related to gadolinium injection. An eGFR below 30 mL/min/1.73 m² is the recommended threshold where gadolinium should not be administered. If the patient is medically unstable, it is prudent to have an eGFR result that is more recent than 6 weeks before the test, as kidney function might have deteriorated in the meantime. See Gadolinium Contrast Medium (MRI Contrast Agents) for more information about gadolinium and kidney function assessment.

MRI scans have safety implications for metal implants of any type in the body when entering a high-strength magnetic field. Referrers should ensure patients presenting for cardiac MRIs have details of the brand, model number and other identifying data of any implants. This will allow the MRI technician to confirm compatibility and maintain patient safety in the MRI environment.

Additional information for MRI angiogram, RV and LV function MRI and structural assessment – Most often the referral is from a cardiologist or cardiothoracic surgeon, and usually as a follow up to a preceding investigation, such as echocardiography. Cardiac MRI is particularly suited to those with congenital heart disease.

What are the absolute contraindications for a cardiac MRI?

• Allergy to gadolinium contrast medium (note not all studies require contrast).
• Non-MRI compatible implants/foreign bodies (e.g. non-MRI compatible pacemaker, large pieces of shrapnel).

What are the relative contraindications for a cardiac MRI?

• Pacemaker/defibrillator – many modern pacemakers are now MRI compatible, but require an on-site technician to put the device in a ‘safe’ mode and to check its subsequent function. Non-MRI compatible pacemakers remain an absolute contraindication;
• Pregnancy (although not usually a consideration in the usual patient population that presents for this test);
• Obesity beyond the limits of scanner table capacity/magnet bore dimensions (this varies with the scanner, so needs to be checked at the time of the appointment being made);
• Claustrophobia/anxiety,
• Severe renal impairment (eGFR <30 mL/min/1.73 m²) (see Gadolinium Contrast Medium (MRI Contrast Agents)).

What are the adverse effects of a cardiac MRI?

In the absence of contrast medium administration, and following appropriate safety screening, there are no anticipated adverse effects of MRI. Sometimes patients may feel a little warm during the scan but this is of no clinical significance.

Gadolinium injection may be required when Cardiac MRI is performed for the following indications:

• Cardiac muscle viability;
• RV and LV functional studies;
• Stress perfusion for cardiac ischemia;
• MR angiography of the great vessels.

The likelihood of adverse events related to the gadolinium injection is very low. Allergic reactions can occur with gadolinium, ranging from mild (e.g. rash) to severe (e.g. anaphylaxis), but this is very uncommon. Any adverse (anaphylactoid or allergic) reaction to gadolinium would be handled within the hospital or imaging department. See Gadolinium Contrast Medium (MRI Contrast Agents) for details about the nature and frequency of anaphylactoid reaction and NSF.

Additional information for stress perfusion MRI

It is more common to experience side effects from the adenosine infusion rather than the gadolinium. These include facial flushing, chest tightness, jaw discomfort, shortness of breath, light headedness and palpitations. These side effects are usually short lived due to the short half-life of adenosine.

Severe side effects are uncommon (incidence approximately 1 in 1500), but include:

• Asystole – Might require drugs or a temporary pacing wire;
• Ventricular tachycardia – Might require treatment with cardioversion;
• Severe chest pain;
• Heart attack;
• Severe shortness of breath.

The risk of death related to adenosine infusion is approximately 1 in 10,000.

Are there alternative imaging tests, interventions or surgical procedures to a cardiac MRI?

Cardiac viability MRI:

• Echocardiogram – Accuracy is dependent on the expertise of the sonographer or technical staff and the cardiologist interpreting the echocardiogram.
• Stress sestamibi or thallium myocardial nuclear perfusion scan – Well established clinically to provide physiological rather than anatomical information, but does lack spatial resolution. Some difficulty in interpretation with ‘balanced’ cardiovascular disease.

RV and LV function MRI and structural assessment MRI:

• Echocardiography – More readily available and likely to be better tolerated. It is better able to assess valvular function compared with MRI. Can be limited by poor acoustic windows and difficultly in viewing some regions of the heart, sometimes requiring trans-oesophageal echo.
• Nuclear medicine – This has lower spatial resolution than MRI, but is widely used to assess heart muscle function.

Stress perfusion MRI:

• Stress echocardiogram – Accuracy is dependent on the expertise of the sonographer or technical staff and the cardiologist interpreting the echocardiogram.
• Stress sestamibi or thallium nuclear scan – Well established clinically to provide physiological rather than anatomical information, but does lack spatial resolution. Some difficulty in interpretation with ‘balanced’ cardiac vascular disease.
• Exercise stress test – Lower accuracy and not suited to all patient populations, e.g. poor exercise tolerance due to arthritis or pulmonary disease.

MRI angiogram:

• CT angiography – Main disadvantages are the radiation dose required for the study, and inability to quantify flow (e.g. valvular regurgitant fraction, shunt calculation) as well as iodinated contrast, which is usually mandatory and might be deleterious in patients with poor renal function (see Iodine-containing contrast medium). Soft tissue contrast is inherently less than MRI. CT does offer excellent spatial resolution, and better demonstration of calcification. Both CT and MRI might be contraindicated in the setting of renal impairment or allergy to iodinated contrast or gadolinium respectively (see Contrast Medium: Gadolinium versus iodine in patients with kidney problems).
• Echocardiography – Safe and more readily available, but offers only limited views of the aorta and great vessels.
• Catheter angiography – More invasive, with greater risk of complication. Offers the best spatial resolution and the potential for endovascular intervention if appropriate. Can be difficult to perform/interpret in the setting of complex anatomy, in the absence of prior cross-sectional imaging.

*The author has no conflict of interest with this topic.

Page last modified on 26/7/2017.